Battle for the Bosporus is the first HoI4's DLC which doesn't add features (even 'Death or Dishonor' brought a few but interesting mechanics regarding equipment), so it's as completely optional as actually recommendable.

It adds their own national focus trees to three countries (Turkey, Greece and Bulgaria), which allows you to take another approach to WWII, as there's lot of 'alternate history' options for the three countries (like trying to reinstate the old Ottoman or Byzantine empires, joining one of the main factions in WWII, etc.). Yugoslavia and Romania are also revisited, but you'll need the aforementioned Death or Dishonor to enjoy their own trees.They are all quite interesting.

With all this game content (decisions, events, focus....) it also adds some new graphics for these countries, and three new music tracks, which are really good.

In conclusion, a good addition to Hearts of Iron IV which expands the numbre of countries that are interesting to play.

Doesn't change mechanics, but adds flair and flavor to the game we all love. More focus trees, more releasable and formable nations.

If you aren't trying to restore the Ottoman Empire or Byzantium, what are you even doing?!

$9 is a no-brainer